1. Field of the Invention
This invention regards a procedure to connect an asynchronous generator on an alternating current and an electrical connecting for use at this procedure. The procedure is further determined by a polyphased generator on a polyphased alternating voltage network which is based on thyristors or similar variable (adjustable) coupling elements, where the connecting is made by means of an adjustable electrical connecting. The connecting degree of the connecting elements can be diversified during a controlled connecting period, which is operated in consideration of the wish of a "soft" connecting on the network and a limit of the maximum amperage during the connecting period, the procedure of which consists more precisely in undertaking a continuous determination of the phase angle .phi. of the generator during the whole connecting period. On basis of the determination a continuous adjustment of the connecting degree of the coupling elements is made (aperture angle, when thyristors is the question). In this way the aimed, successive load connecting of the generator is reached on the network (soft connecting), and the procedure includes furthermore a method to determine the generator phase angle .phi. by registering the time of zero passage of the phase voltage and a simultaneous detection of the respective thyristor power-zero passage period. As the difference between the two periods indicates an expression of the phase angle .phi. in the respective phase.
The connecting can be an electrical connecting for the discharge of the procedure.
2. Description o f Related Art
If an asynchronous motor is driven below the synchronous orbital number, the machine will operate as an electrical generator transferring the induced mechanic axle output to electrical power. Provided that the machine--now as an asychronous generator--is connected to an alternating voltage network, the generated current will be sent into this. This characteristic is used in fashion wind turbines and similar energy systems for utilisation of stochastic energy sources. It is to be noticed that to this purpose asynchronous machines developed specially to generator running used.
Especially, in connection with wind turbines, the connecting of the network involves large difficulties due to the unstable nature of the wind. If one imagined that the wind speed can be adjusted as another controllable operating parameter, one could slowly increase the wind until the rotor of the wind turbine is only rotating with a speed corresponding to synchronous generator orbital number. When the phases are adjusted in synchronous, the connecting of the network can be made by means of a relay or a similar simple electrical contact machine without involving pulse on the network or torque transitions in the turbine system (i.e. rotor, gear and generator). But it is not that simple in the real world.
In practice one naturally has to take account the varying speed and inconstant, incalculable nature of the wind. It is a serious problem in connection with wind turbines that especially the speed wind and hereby the contents of energy can change very fast. Therefore, at strong gusts, the wind turbine rotor can be sped up below the synchronous orbital number in a very short time, provided that one initiate from a no-load state, where the turbine rotates with under-synchronous speed, and when the generator therefore is unloaded. The connecting of the generator to the network has to happen in this very moment when the synchronous speed is passed, or the wind turbine will bolt. It is necessary to undertake the connecting "soft" in order to provide unacceptable big pulses of the network and torque pulses of the rotor during the connecting course. It means that during the connecting period the power transmission between the generator and the network has to be increased gradually from a miminum until completely connected to power. This demand is, however, contradicted to the demand of a fast connecting of the generator on the network, when the dynamic conditions around the turbine demand this, i.e. when the wind turbine is exposed for strong gusts as described. In practice, it is necessary to choose a compromise, where the connecting of the generator is maintained during a controlled connecting period, thus the effect partly on the network, partly the construction of the wind turbine as such (torque load of gear and rotor shaft, influences of bends on the wings etc.) are kept within acceptable limiting values. For this purpose, different types of electrical connectings and different generator connectings and controlling principles are developed.
The basis of the connecting adjustment in the connection known is for instance measuring of resulting generator voltage. Here the question of making connecting adjustment on basis of an indirect measuring, as the generator voltage and the generator current intensity depend on many factors. Therefore one cannot work out a precise adjustment without resulting in moment transitions between the network and the turbine during different operating conditions, especially during the connecting period. At such known adjustments, it can be difficult to keep the moment transitions, and from these following pulses in the network within the limits made by the owner of the network. It is especially difficult to decrease the moment transitions, when the generated effect is small in proportion to the normal power of the generator.
An electrical connecting is described below, where the connecting of the network is carried out by means of thyristors or similar variable, adjustable connection elements known from the description to U.S. Pat. No. 4,656,413 to Bourbeau. The aperture angle of the thyristors--and hereby the connecting degree--is controlled by means of measuring of the generator voltage and the generator current. Beyond this the revolution number of the generator is measured with a revolution counter for determination of safety limits, and the system has further more means for measuring of the wind speed. When the wind speed is below a fixed level, the electrical connection between the network and the generator is disconnected. In practice, it has appeared from the connecting and the way of operation that the consumption cannot be under 4-8%, especially to excitation of the asynchronous generator. When the wind turbine is in low wind position, i.e. when the turbine is connected to the network and the generator only produces miminal power, then the consumption is still 4-8% of the maximum effect, which the generator can produce. It is clear that this is unacceptable from an economic point of view. Beyond this the electrical connecting indicates a controlling form, where the generator is disconnected by low wind operation to avoid that the network pulls the generator as a motor. The result of this includes repeated connecting and disconnecting during this operation state. Every connection involves unwanted torque impacts on the rotor system and a pulse on the network.
Regardless of the gradual increase of the thyristor aperture angle after a programme prepared in advance (ramp) and due to this following gradual connecting degree on the network during the passage of the asynchronous generator of the asynchronous circulation number, an unwanted great big pulse appears in the system during the connecting period. The easiest way to explain this pulse is that the phase angle .phi. of the asynchronous machine increases significantly, when the machine converts from motor running until generator running, i.e. during the passage of the synchronous rotation number. The phase angle .phi. affects the efficient thyristor opening time, thus the opening time increases, when .phi. increases (provided that the thyristor opening angle is unchanged). It means that a gradual increase of the thyristor opening angle not alone is enough to ensure a "soft" connecting process without pulses. It is said in a popular way that the thyristor connecting is self-increasing in a narrow rotation interval around synchronous rotation number. A more intelligent control of the thyristors is necessary, where among several operation parameters it is necessary to pay regard to changes in the phase angle .phi. during the process, if an intended limit of the amperage is to be achieved during the connecting process and immediately after this.
The wish of a limit of the connecting strength is not only due to the technical condition about the turbine construction--here one thinks of a prolonged mechanic service life of gears and wings, etc., when the torque impacts are limited--but also due to the fact that today several network owners demand that the wind power system current connected complies with the further specified demands of a maximum connecting amperage. The purpose of the present invention is just to show a connecting principle and a connecting meeting this demand.